In turbomachinery, the quest for increased efficiency often leads to rotor designs which are quite "flexible," i.e., the rotors are either slender or have a relatively long span between the bearings supporting the rotor or both. These flexible rotors tend to have natural frequencies which are lightly damped, which results in excessively large vibration amplitudes when the rotor operates near critical speeds. Also, flexible rotors tend to be more susceptible to whirl instability induced by either bearing oil film forces or aerodynamic forces.
The damping factor associated with the natural frequencies of flexible shafts can be greatly enhanced if the dynamic stiffness of the elements supporting the shaft can be reduced, while still retaining a high damping coefficient in the shaft supports. Providing a soft support for journal bearings in turbomachinery is a very effective means of increasing the damping provided to rotor vibrations and also a very effective means of stabilizing the rotor against self-excited whirl produced by bearing film forces or aerodynamic forces. Soft supports for bearings can also greatly reduce the "noise" transmitted from rotor vibrations to the rotor support base. However, it is usually necessary to retain a high static stiffness in shaft support elements to position the shaft accurately within shaft seals and to accurately control the position of turbine blades within their enclosures. Consequently, there has been a need to develop a support system for a rotor which has a high static stiffness but a low dynamic stiffness. That is, the support system should provide a high resistance to steady state displacements of the shaft and low resistance to dynamic vibrations of the shaft. Typical bearings, whether of the fluid film or rolling element type, do not provide this type of support, since their stiffness values are the same for static and dynamic motions. The only shaft support systems known to applicant which have high static stiffness combined with low dynamic stiffness are those obtained by the use of magnetic bearings. Magnetic bearings, however, have many disadvantages. They are relatively costly, require complex control systems and require redundant backup bearings in the event of power failure.